1. Field of the Invention
This invention relates to the use of ignition accelerators to improve the cetane number and ignition properties of diesel fuels. More particularly, it relates to the use of N,N-disubstituted organic nitroxides as additives to increase the cetane number of a liquid hydrocarbon diesel fuel.
2. Description of the Prior Art
During the operation of a diesel engine, there is a delay period between the time of fuel injection and fuel ignition, which is referred to as the ignition delay period. Ignition of the fuel then results in a rapid pressure increase which serves to drive the engine. If the ignition delay period is too long, the subsequent ignition can result in a pressure increase which is too rapid for proper engine operation. Indeed, the pressure increase can become so rapid that severe knock and engine damage can occur. Alternatively, if the ignition delay period is too short, there is inadequate time for the mixing of air and fuel which can result in the formation of smoke.
The ignition delay period is influenced to some degree by engine operating conditions such as injection timing, compression ratio, rate of fuel injection, and inlet air temperature. In addition, the ignition delay period is highly dependent upon the fuel composition. The cetane number of the fuel is a measure of the influence that the fuel exerts on this parameter. In more specific terms, the cetane number of a fuel is a number which is equal to the volume percent of normal cetane in a blend with 1-methylnaphthalene which matches the ignition properties of the fuel. That is to say, a scale is used wherein normal cetane is given a number of 100 and 1-methylnaphthalene is assigned a number of 0.
As a broad generality, aromatic hydrocarbons usually have low cetane numbers while paraffins tend to have high cetane numbers and naphthenes are intermediate between the aromatics and paraffins. Typical refinery streams would commonly be expected to possess cetane numbers in the following ranges: (1) heavy gas oil, 50-55; (2) light gas oil, 44-49; (3) heavy naphtha, 41-46; and (4) cracked gas oil, 17-31. It will be appreciated, of course, that these ranges are merely approximate and are dependent upon the precise chemical composition of the refinery stream.
A satisfactory diesel fuel should ordinarily have a cetane number which is not less than about 40 in order to insure proper ignition in a compression ignition engine. This requirement presents no problem so long as adequate volumes of high cetane number distillates are available. However, the supply of such high cetane number material is relatively inflexible. Therefore, as the demand for diesel fuel continues to increase, a corresponding need exists to incorporate fuels into the diesel pool which have cetane numbers that are too low for satisfactory use in a compression ignition engine. One method of accomplishing this involves the use of additives which serve as ignition promoters and increase the cetane number of the fuel.
A multitude of materials have been proposed in the prior art for use as ignition improvers. Included in the group of materials which have previously been proposed for this purpose are the following: (1) hydrocarbons such as acetylene, divinylacetylene and butadiene; (2) aldehydes, ketones, ethers and alcohols such as furfuraldehyde, acetone, diethyl ether, ethyl acetate, nitroglycerine and methyl alcohol; (3) metal derivatives such as barium nitrate, copper oleate, manganese dioxide, potassium chlorate, and vanadium pentoxide; (4) alkyl nitrates and nitrites such as ethyl nitrate and ethyl nitrite; (5) alkyl and aromatic nitro compounds such as nitroethane, nitrobenzene and nitronaphthalene; (6) oximes and nitroso compounds such as formaldoxime and nitrosomethylmethane; (7) peroxides such as acetone peroxide; (8) polysulfides such as diethyltetrasulfide; and (9) borine trialkyl amines. Unfortunately, many of these prior art materials are relatively ineffective as ignition improvers. However, peroxides and alkyl nitrates are highly effective.
N,N-disubstituted organic nitroxides are organic free radicals which, depending upon structure, have a stability which can vary over a wide range. For example, piperidine-1-oxyl is unstable whereas 2,2,6,6-tetramethylpiperidine-1-oxyl is a stable compound. In general, N,N-disubstituted organic nitroxides having no .alpha.-hydrogen atoms are stable compounds. The preparation of N,N-disubstituted organic nitroxides is described in the following references: "Free Nitroxyl Radicals" by E. G. Rozantsev, Plenum Press, New York, N.Y., 1970; "Organic Chemistry of Stable Free Radicals," by A. R. Forrester et al., Academic Press, New York, N.Y., 1968; and U.S. Pat. Nos. 3,163,677; 3,334,103 and 3,502,692.
2,2,6,6-Tetramethyl-4-oxopiperidine-1-oxyl and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl have been proposed as pour-point depressants for fuels and lubricants in U.S.S.R. Pat. No. 630,284. It is disclosed that these materials can be added to lubricants and fuels in amounts of from 0.04 to 0.2 weight percent for this purpose. However, this patent fails to either suggest or disclose the use of N,N-disubstituted organic nitroxides to improve the cetane number of diesel fuels.
U.S. Pat. Nos. 3,163,677; 3,334,103; 3,502,692 and 3,759,926 disclose that N,N-disubstituted nitroxides are useful as antiknock agents in fuels. These patents do not, however, either suggest or disclose the addition of these nitroxides to diesel fuels. Antiknock agents serve to increase the octane number of a fuel and are believed to function by combining with the reactive intermediates which are produced during hydrocarbon combustion and thereby destroying the ability of these intermediates to propagate further oxidation which leads to knock. Antiknock agents retard combustion whereas, in contrast, cetane improvers serve to promote combustion. Consequently, any increase in the octane number of a hydrocarbon fuel will be reflected by a corresponding decrease in the cetane number of the fuel. Indeed, this relationship can be approximated by the Wilke equation: EQU Cetane Number=60-0.5(Motor Octane Number)
This inverse relationship between cetane number and octane number is discussed, for example, by K. Becker in "The Influence of an Ignition Accelerator on the Ignition Quality and Anti-Knock Properties of Light Hydrocarbons in the Diesel Engine," Society of Automotive Engineers, Paper No. 760163, 1976. As a consequence, the literature teaching that N,N-disubstituted nitroxides are antiknock agents is also a teaching that these materials cannot be used to improve the cetane number of a fuel.